BTW, Gray was pulling a con job over Avatar. He literally acted like he was blood thirsty and out for revenge, which caused the Avatar member to think his obsession for revenge was making his soul and body black. It wasn't. He lied. His markings can be undone, and he was working with Erza the whole time.
Gray only really tapped into his darkness when he thought Juvia died, and he didn't have to demonize to beat Invel.
that is combining his magic with Lucy, so that is both of their strength, excuses. And Black Magic is strengthened by the darkness of the heart, which is the point Invel was making, Darkness in an individual gives rise to BM. And? you have not shown her doubting her humanity, nor giving into darkness prior or in her transformation as she was acting out of love for her child despite her situation. Natsu's seed stopped because of Anti-Bodies, So then you agree that the darkness was nonsense for Dragon Seed, besides, not the same as Black Magic.
He didn't combine his magic with Lucy. He used his magic on the doll, which gave Lucy some of his flames. He didn't combine his magic powers with Lucy. Lucy just used some of his flames to hit Kain Hikaru with.
I don't need to show Irene doubting her humanity. Doubting his humanity was Natsu's darkness. Irene's darkness may not necessarily be the same emotions. The fact she has a dragon seed at all is proof enough of her having darkness, simply because Igneel said explained the dragon seed were seeds of darkness.
Look, the question
@Jko had was whether or not any villains can power up using their emotions, and the answer is that they obviously do. You have a bunch of them. Zeref, Acnologia, Irene, Hades, and every other black mage... All of them can tap into the darkness in their hearts as a source of power.